Modern internal combustion engines frequently have a system for external exhaust gas recirculation (AGR). By this, one should understand a pipe connection between the exhaust gas pipe and the intake manifold which is able to be controlled using an exhaust-gas recirculation valve (AGR valve) to be open and closed, and which may be equipped with a pump device. In this pipe connection, combusted exhaust gas flows back into the intake manifold, either driven by a gradient between the exhaust gas counterpressure and the intake manifold pressure or by the pump device. The variant without the pump device occurs far more frequently.
If the internal combustion engine is outfitted with an exhaust gas turbocharger, then, as a rule, the exhaust point of the exhaust gas is upstream of the turbine and the supply point to the intake manifold is downstream from the compressor. A supply point to the intake manifold upstream of the compressor is also conceivable, however. The admixture of exhaust gas to the aspirated air is able to improve the efficiency and/or the raw emission of the internal combustion engine.
The legal emission regulations and on-board diagnostic regulations, in the most important vehicle markets, make the use of oxygen sensors in the exhaust tract of the internal combustion engine unavoidable.
In this invention, the term oxygen sensor designates every type of gas sensor which emits a signal that correlates with the concentration of oxidizing and reducing gas components. The ratio of oxidizing to reducing gas components before combustion is defined by the lambda value. In a motor operation having a sufficiently high excess of air (lean operation), there are no reducing components present in the exhaust gas, and there is almost only oxygen present as oxidizing component. That is why measuring lambda at these operating points means the same as measuring the oxygen concentration.
Oxygen sensors for internal combustion engines, that are common in the trade, were developed for use in the exhaust tract and may usually be developed as lambda probes, whose primary measured quantity is a lambda value, or as an NOx sensor, whose primary measured quantity is the NOx concentration, but which is also able, besides that, to supply a lambda signal.
The signals of the oxygen sensors are usually supplied to an engine control unit, which regulates the operating parameters of the internal combustion engine, on the basis of this and additional measured values. Furthermore, in the engine control unit, methods are implemented for the optimal operation and the on-board diagnosis of the oxygen sensors. The following methods are a part of this:                A method for generating a further signal of the oxygen sensor that is correlated with the sensor temperature. This may be a resistance value, for example.        A method for controlling and regulating the heating of the oxygen sensor, with the aim of holding the sensor in the optimal temperature window. For this purpose, the above cited temperature signal is used.        A method for compensating the temperature dependence of the oxygen sensor, in case the sensor temperature deviates from the target temperature, i.e. from the value for which the sensor characteristics curve is specified.        A method for compensating for the pressure dependence of the oxygen sensor, in case the pressure at the location of installation of the sensor deviates from the value for which the sensor characteristics curve is specified.        A method for characteristics curve compensation. In this case, at special operating points at which the oxygen concentration is known independently of the sensor signal, the sensor signal is compared to the signal value specified for this oxygen concentration, and from the comparison a correction value is ascertained, using which the sensor signal is corrected as a result. Among the known methods, the oxygen concentration known independently of the sensor signal, is the concentration value in pure air. If the sensor is installed in the exhaust tract, however, the operating points, in which the oxygen sensor is exposed to pure air, are naturally seldom and brief. Almost all known methods are supported for this on the so-called overrun condition. One problem is that an overrun condition cannot be forced by the engine control, but is a function of the driver's torque command.        
Oxygen sensors in the intake manifold of internal combustion engines are discussed, for instance, in DE 2744844 A1 and from US 2007/0044472 A1. The external exhaust gas recycling is regulated based on the signal of this sensor.
In that manner, DE 2744844 A1 discusses a method for regulating the air/fuel ratio of a mixture supplied to an internal combustion engine, it being provided that, in the intake manifold of the internal combustion engine using a sensor, one should scan a parameter which is representative for the air/fuel ratio of the mixture, and during the first state, at which the temperature of the sensor is below a specified value, above which sensor is operable, one heat the sensor, that one break off the heating of the sensor after a temperature is reached above the specified value and that one should regulate the air/fuel ratio of the mixture produced for the internal combustion engine to a desired value, by regulating the flow throughput of the fuel supplied to the internal combustion engine for the production of the mixture. An appropriate apparatus for carrying out the method is also described.
It is true that use of oxygen sensors in the intake manifold occurs substantially less than the use in the exhaust tract of the internal combustion engine. For this reason, for the operation of oxygen sensors in the intake manifold, not so many highly developed methods exist as yet as for operation in the exhaust tract.
Thus, DE 102005056152 A1, for example, discusses a method and a corresponding device for implementing the method, for calibrating a lambda measuring signal provided by a broadband lambda sensor situated in the exhaust gas region of an internal combustion engine, in which a correction value for ascertaining a measure for the actual lambda value is drawn upon, the correction value being ascertained during a specified operating state of the internal combustion engine, in which no fuel metering takes place, and the rotational speed of the internal combustion engine is above a threshold value, the correction value being ascertained as a function of the sensor temperature of the broadband lambda sensor during the specified operating state. Consequently, this method describes a method for learning the temperature dependence of lambda probes in the exhaust track.
For a series of methods for the operation in the exhaust gas pipe, as was described above, for instance, the differences in the environmental conditions in the intake manifold from those in the exhaust tract are relevant. In some cases, the environmental conditions in the intake manifold prepare particular difficulties that require new solutions. However, in some cases they also offer possibilities for methods which are not possible in the environmental conditions in the exhaust tract, although they would be desirable to function there too. In the operation of an oxygen sensor in the intake manifold or generally in the air supply channel, a particular challenge is posed especially by the temperature dependence.
Since the oxygen sensor in the air supply channel is exposed to cold air, and not to hot exhaust gases, there is an increased possibility that the performance of the sensor heating is not sufficient